In order to prevent a metal material from the environmental corrosion during use, application of various coating materials has been heretofore employed. The coat layer formed on the metal sheet usually uses an electrically non-conducting resin as the binder and therefore, this layer has no electric conductivity and can be hardly welded and earth-grounded. To solve this problem, a technique of forming a coat layer containing electrically conducting particles on a metal sheet to impart electric conductivity and thereby enable welding or earth-grounding has been proposed.
For example, Japanese Unexamined Patent Publication (Kokai) No. 09-234820 discloses a technique of imparting weldability by subjecting the metal sheet to coating with a resin using iron phosphide as the electrically conducting particles. In this method, the amount of the electrically conducting particles is specified to be 20 to 45 mass % and the weldability is ensured by controlling the amount to this range. As for the particle size, it is stated that the particles preferably have an average particle size of 20 μm or less.
Japanese Unexamined Patent Publication (Kokai) No. 07-314601 discloses a technique of imparting an earthing property by using nickel-based particles as the electrically conducting particles. In this method, the average value and the maximum value are specified with respect to the particle size of the electrically conducting particles and it is stated that the matter of importance for ensuring electrical conductivity is to add, in the case of a scale-like particle, from 11 to 200 parts of the electrically conducting particles having a long diameter of 100 μm as a maximum and 15 μm on average and in the case of a chain-like particle, 10 parts or less of the electrically conducting particles having a maximum particle size of 44 μm and an average particle size of 2.5 μm.
Also, in Japanese Unexamined Patent Publication (Kokai) No. 01-60668, the ratio of the average particle size of metal-based particle to the thickness of coating film is specified for imparting the electrical conductivity and it is stated that when the average particle size is from 0.5 to 3 times the film thickness, electric conductivity can be ensured. The particle size is not particularly described but in Examples, a particle having an average particle size of 7.5 to 25 μm is used.
Recently, Japanese Unexamined Patent Publication (Kokai) No. 2002-172363 has proposed a technique of coating an organic resin film containing from 10 to 70 mass % of ferrosilicon having a particle size of 0.5 to 10 μm on a zinc-plated steel sheet to a thickness of 2.5 to 8 μm to obtain a surface-treated steel sheet having excellent weldability.
The techniques disclosed in these publications are satisfied in terms of imparting electric conductivity to the coat layer and thereby ensuring weldability or earth property as a coated metal sheet, but are still insufficient from the aspect of obtaining satisfactory formability and corrosion resistance as well as stable weldability and an earthing property. One of the causes therefor is in that only the concept of average or maximum particle size is used in designing the particle size of the particle but the particle size distribution is not taken into consideration.
Japanese Unexamined Patent Publication (Kokai) Nos. 09-234820 and 2002-172363 supra disclose that a rust-preventive pigment is added for the purpose of enhancing the corrosion resistance. However, when a rust-preventive pigment is added in addition to the electrically conducting pigment, the electric conductivity or formability decreases and therefore, if possible, the addition of a rust-preventive pigment must be minimized. Japanese Unexamined Patent Publication (Kokai) No. 2002-172363 also discloses that when a zinc alloy-plated steel sheet is used for the underlying plated steel sheet, a coated steel sheet having more excellent corrosion resistance can be obtained. However, a change in the kind of plating may cause increase in the cost or may sacrifice other performances and therefore, it is demanded to obtain high corrosion resistance or formability independently of the kind of the underlying steel sheet.
In order to solve these problems, a first object of the present invention is to provide a coated metal sheet excellent in electric conductivity (for example, weldability and an earthing property), corrosion resistance and formability.
In a metal-made part for the outer panel of automobiles, the joined portion or folded hem portion on the inner surface of a hollow part prone to severe corrosion is coated by electrodeposition painting and/or applied with a rust-preventive subsidiary material such as sealer, adhesive and wax, whereby the corrosion resistance is ensured in many cases. On the other hand, studies are being made to reduce the production cost of an automobile by omitting or decreasing the painting on the inner surface of an automobile part or the rust-preventive treatment with sealer-wax or the like, which is performed for the purpose of rust prevention, and various coated steel sheets have been heretofore proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 55-17508 discloses a technique of forming a zinc-containing coating film on a steel sheet surface. Also, a steel sheet capable of ensuring rust perforation resistance even when electrodeposition painting or rust-preventive subsidiary material is not applied has been proposed and, for example, Japanese Unexamined Patent Publication (Kokai) Nos. 9-23480, 10-128906 and 11-5269 disclose a steel sheet where an electrically conducting resin coat layer is formed on a steel sheet surface. As for the constitution thereof in general, an organic film containing an electrically conducting or rust-preventive pigment is coated on a plated steel sheet through an undercoating layer.
However, when the steel sheet coated with such an organic film is formed as an automobile part through severe drawing or bending of press forming, the corrosion resistance in the formed part may decrease. One of the reasons therefor is because the film cannot follow the deformation of steel sheet and this causes cracking or separation of the film. Also, when the film is made thick so as to ensure sufficiently high corrosion resistance, this causes problems that welding and, in turn, stable production become difficult and the cost increases. Accordingly, conventional coated steel sheets decrease in the corrosion resistance or must receive a repair coating.
A second object of the present invention is to provide a weldable coated metal material in which, even when the steel sheet is greatly deformed by the forming under severe conditions, sufficiently high corrosion resistance can be ensured in the formed part.
In fuel tanks of automobiles, excellent formability (deep-drawing property) is required because the fuel tank has a complicated form in many cases. Also, the fuel tank is an important safety part of an automobile and therefore, it is essential that the material used therefor generates no corrosion product giving rise to filter clogging, is freed from corrosive perforation, and can be easily and stably welded. As for the material having these various properties, a lead-tin based alloy-plated steel sheet (see, Japanese Examined Patent Publication (Kokoku) No. 57-61833) has been heretofore widely used as a material for the fuel tank of automobiles. This material has a stable chemical property against gasoline, excellent press formability by virtue of high lubricity of the plating, and also excellent resistance weldability such as spot welding and seam welding. However, in view of environmental impact, materials not using lead are demanded and there are disclosed techniques such as tin-based alloy-plated steel sheet (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-269733), aluminum-based alloy-plated steel sheet (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-156027), metal- or organic film-coated zinc-based alloy-plated steel sheet (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 08-296834).
In recent years, enhancement of the rust-preventive fuel tank to an extent of assuring no perforation over 15 years is demanded in North America and with this requirement, there arises a problem that corrosion resistance by conventional techniques is insufficient. When a tank is produced through forming and welding and then applied with multi-ply coating, long-term rust prevention can be achieved, but in this case, a problem of incurring a great increase in the cost arises. Also, for example, Kokai No. 6-306637 discloses a technique of applying a metal powder-containing organic film to an aluminum-based alloy-plated steel sheet, but this method has a problem in that, when the steel sheet is shaped through severe drawing or bending of press forming, the film cannot follow the deformation of steel sheet and this may cause cracking or separation of the plating layer or the film to decrease the corrosion resistance of the formed part or in that when the film is made thick so as to ensure sufficiently high corrosion resistance, the welding and in turn the stable production become difficult and the cost increases. Furthermore, Japanese Unexamined Patent Publication (Kokoku) No. 3-25349 discloses a technique of coating a zinc-plated steel sheet with an organic film containing various metal powers, but the zinc-based plating has a fear of causing filter clogging due to generation of corrosion product ascribable to an organic acid and coagulated water produced by gasoline degradation particularly in the portion subjected to severe forming. In addition, as the outer surface is also required to have high corrosion resistance, an organic coating film having a larger thickness must be applied to both the inner surface and the outer surface and this may cause problems that the welding and, in turn, the stable production, become difficult and the cost increases.
A third object of the present invention is to overcome the above-described problems in the performance and production and provide a rust-preventive steel sheet capable of realizing, as a fuel tank, high inner surface corrosion resistance and exhibiting good formability and stable weldability.